A novel permanent magnet of R(referring at least one element of rare-earth elements)-Fe-B system has been proposed (in Japan Patent Application Laid-Open No. Sho 59-46008, and Japan Patent Application Laid-Open No. Sho 59-89401), which is made of mainly rare-earth elements rich in Nd or Pr and B and Fe (eventually, therefore, the R-Fe-B system magnet does not contain expensive elements such as Sm or Co) and has superior magnetic characteristics to those found in the conventional type of rare-earth cobalt magnets.
Although the Curie point of the aforementioned magnet alloy is reported, in general, to be in a temperature range from 300.degree. C. to 370.degree. C., the Curie point of the R-Fe-B system permanent magnet (Japan Patent Application Laid-Open No. Sho 59-64733 and Japan Patent Application Laid-Open No. Sho 59-132104) was improved to be higher than that reported for the conventional type magnet by substituting a portion of Fe element by Co element. Moreover, in order to develop a new type of permanent magnet having an equivalent or higher Curie point and higher maximum energy product, (BH)max, than the aforementioned Co-containing R-Fe-B system permanent magnet and to improve the temperature characteristics, particularly intrinsic coercive force, iHc, another new type of Co-containing R-Fe-B system permanent magnet has been proposed (Japan Patent Application Laid-Open No. Sho 60-34005), in which the intrinsic coercive force iHc can be enhanced by maintaining an extremely high value (BH)max of more than 25MGOe, by substituting a compositional fraction of R (which mainly represents light-weight rare-earth elements such as Nd or Pr) in the Co-containing R-Fe-B system permanent magnets by at least one element chosen form the element group comprising of heavy-weight rare-earth elements including Dy or Th.
Conventionally, the ferrite magnet has been employed as a magnet used in a vacuum atmosphere with an order of 10.sup.-3 Pa. However, the ferrite magnet has relatively low magnetic properties, which are not high and sufficient enough to employ to the undulator.
There are several important items required for a satisfactory permanent magnet used for ultra-high vacuum atmosphere of lower than 1.times.10.sup.-9 Pa; they include
(1) excellent magnetic characteristics, PA0 (2) no generation nor exhaustion of absorbed or contaminated gas from the magnet surface, and PA0 (3) maintaining the high level of vacuum of 1.times.10.sup.-9 Pa even after the magnet being installed to the relevant equipment.
Accordingly, the aforementioned R-Fe-B system magnets could have been applied to the undulator used in the ultra-high vacuum because of their high magnetic properties. However, since the gas can easily be adsorbed on or absorbed in the R-Fe-B system magnets, the adsorbed or absorbed gas will be generated or exhausted from the magnet surface layer, causing a difficulty in maintaining the ultra-high vacuum of less than 1.times.10.sup.-9 Pa. As a result, the conventional type of R-Fe-B system permanent magnet cannot be used for the ultra-high vacuum atmosphere. In a case when the R-Fe-B system magnet, on which Ni-plating was surface-treated for an anti-corrosion purpose, is utilized in the ultra-high vacuum, the magnet cannot be placed inside the vacuum chamber, rather it is installed outside thereof in order to build the undulator or the similar device. Accordingly, the equipment itself becomes much larger size and the excellent magnetic properties found in the R-Fe-B system magnet cannot effectively be practiced.
Even with other types of R-Fe-B system magnets with which various metals or polymeric resins are coated in order to improve the corrosion resistance of the R-Fe-B system magnets, the generation or exhaustion of adsorbed/absorbed gas is unavoidable, resulting in that the usage of such corrosion-resistant R-Fe-B system magnet is very limited for the ultra-high vacuum atmosphere of, particularly, lower than 1.times.10.sup.-9 Pa.
It is, therefore, an object of the present invention to provide a permanent magnet having excellent magnetic characteristics which can be employed for the undulator used in the ultra-high vacuum atmosphere. Furthermore, the permanent magnet according to the present invention has a dense and strongly bonded surface coated layer thereon in order to prevent any gas generation or gas exhaustion out of the magnet surface layers; hence the presently invented magnet has a completely different features from the conventional type of corrosion-resistant R-Fe-B system magnet on which various coated film is applied for anti-corrosion purpose.